In recent years, reduction in weight and thickness of a display of a personal computer, a television, and the like is in strong demand and a liquid crystal display apparatus which easily realizes lighter weight and reduced thickness is being rapidly employed as such a display. However, since response speed of liquid crystal is low, there is a case that sufficient picture quality is not obtained when a moving image is displayed in a liquid crystal display apparatus. To suppress deterioration in picture quality at the time of displaying a moving image caused by low response speed of liquid crystal, conventionally, a drive method called overdrive (or overshoot drive) is employed. The overdrive is a driving method of supplying a drive voltage higher than a gray scale voltage corresponding to an input image signal of a current frame or a drive voltage lower than a gray scale voltage corresponding to the input image signal of the current frame to a liquid crystal display panel in accordance with a combination of an input image signal of an immediate previous frame and an input image signal of the current frame. By employing such overdrive, time until voltage reaches the gray scale voltage corresponding to the input image signal of the current frame is shortened, and deterioration in the picture quality at the time of displaying a moving image is suppressed.
In a liquid crystal display apparatus employing the overdrive, a look-up table (hereinbelow, called an overdrive look-up table) is held so that a drive voltage is determined on the basis of a combination of a gray scale value corresponding to an input image signal of an immediately preceding frame (hereinbelow, called “a previous frame gray scale value”) and a gray scale value corresponding to an input image signal of the current frame (hereinbelow, called “a current frame gray scale value”). FIG. 10 is a diagram schematically illustrating an example of an overdrive look-up table held in a liquid crystal display apparatus capable of performing 256-gray scale display. In FIG. 10, the numerical values written in the leftmost column indicate previous frame gray scale values, and the numerical values written in the uppermost row indicate current frame gray scale values. A numerical value written in an intersection position of a row and a column indicates a gray scale value (hereinbelow, called “application gray scale value”) corresponding to a drive voltage determined on the basis of the combination of a previous frame gray scale value and a current frame gray scale value. For example, in the case where the previous frame gray scale value is “64” and the current frame gray scale value is “128”, the application gray scale value is “155”. For example, in the case where the previous frame gray scale value is “160” and the current frame gray scale value is “64”, the application gray scale value is “20”. In such a manner, on the basis of data stored in the overdrive look-up table, the drive voltage higher than the gray scale voltage corresponding to the input image signal of the current frame or the drive voltage lower than the gray scale voltage corresponding to the input image signal of the current frame is applied to the liquid crystal.
As described above, when the overdrive is performed, the application gray scale value is determined in accordance with the overdrive look-up table on the basis of the combination of the previous frame gray scale value and the current frame gray scale value. Consequently, the previous frame gray scale values of one frame have to be held. Accordingly, usually, in the liquid crystal display apparatus employing the overdrive, a volatile memory (for example, SDRAM) for holding the previous frame gray scale values of one frame is provided. It should be noted that such a volatile memory holding the gray scale values of one frame is generally called a “frame memory”.
As the resolution of a liquid crystal display apparatus becomes higher, the capacity of the above-described frame memory has to be increased. When the number of display pixels increases as the resolution becomes higher, the amount of data related to writing to the frame memory and reading from the frame memory in one frame period increases, so that the data transfer speed has to be increased. Consequently, the cost rises. Regarding the liquid crystal display apparatus employing the overdrive, for example, in the invention disclosed in Japanese Patent Application Laid-Open No. 2006-208770, to reduce the capacity of the frame memory, image data is encoded (compressed) and then the encoded image data is written in the frame memory. In an image processing apparatus described in Japanese Patent Application Laid-Open No. 2006-208770, at the time of quantizing image data of a current frame block by block and then outputting encoded image data, the number of quantization bits of the image data is adjusted on the basis of the dynamic range of each block.
In addition to the Japanese Patent Application Laid-Open No. 2006-208770, the following prior art documents related to the present invention are known. Japanese Patent Application Laid-Open No. 2010-2668 discloses a technique of suppressing residual image noise which occurs when input image data is corrected on the basis of image data which is irreversibly compressed and then decoded. Japanese Patent Application Laid-Open No. 2006-267172 discloses a technique of suppressing deterioration in the picture quality while suppressing the memory capacity in a liquid crystal display apparatus performing the overdrive. In addition, Japanese Patent Application Laid-Open Nos. 2008-242472, 2003-345318, 2007-10699, and the like also disclose a technique of reducing the memory capacity in a liquid crystal display apparatus.